Tendon-transfer surgeries are performed to partially restore musculoskeletal function for a variety of conditions such as stroke, paralysis, nerve, muscle, brain, or spinal trauma, and congenital disorders. Surgical techniques for restoring or improving musculoskeletal function typically involve attaching the muscles and tendons to a bone or muscle using sutures. In at least fifteen types of hand tendon-transfer surgeries, a single donor muscle is directly sutured to multiple recipient tendons. The suture couples the movement of the muscle and the tendons to replicate prior function, but cannot preferentially enhance, scale, or distribute a muscle's force and movement across the tendons. This leads to limited post-surgery musculoskeletal function and surgical choices.
Prior work has explored three primary types of medical devices for restoring or enhancing musculoskeletal function: (1) rigid passive implants that directly attach to bones, such as joint replacement implants or implants for holding bones together after fractures; (2) implants that secure two biological tendons, such as tenofix and Otho-Hunter implants, or biological tendons to bone, such as the Orthocoupler implant; (3) external devices that are body-powered or externally powered, such as prostheses for lost body parts, orthoses for correcting misalignments, or exoskeletons for assisting in movement.
Two of the most common types of surgery for the improvement of musculoskeletal function are tendon-transfer surgery and knee-replacement surgery. Each of these conditions severely affects hand function and prevents a person from performing even basic activities of daily life. Tendon-transfer surgery restores lost hand function by re-routing one or more tendons from the affected muscle and suturing them to a functioning muscle. For example, in tendon-transfer surgery for high median-ulnar palsy, a severe condition that disables all four flexor digitorum profundus (FDP) muscle bellies, all four FDP tendons are sutured directly to a functioning muscle, such as the extensor carpi radialis longus (ECRL). The ECRL muscle, which is a wrist extensor, has only one muscle belly. Accordingly, when the ECRL contracts, all four fingers curl inward simultaneously and equally. As a result, if one finger makes contact with an object during the grasping process while the other fingers are still closing, further ECRL contraction to close the remaining fingers forces the finger that has already made contact to curl further and slip on the object. Furthermore, the muscle may have to stretch the tendon of the finger that has already made contact in order to flex the other fingers, increasing muscle force requirement. Overall, this deleteriously affects grasping capability and limits the activities of daily living.
A key problem with the current surgical procedure is that the suture couples the movement of all four fingers, leading to poor hand function in fundamental tasks such as grasping of objects since the fingers cannot adapt naturally to the object shape. Specifically, the coupled finger movement leads to (1) incomplete and weak grasps, (2) greater muscle force requirement since the muscle has to isometrically stretch tendons to flex the other fingers once one finger makes contact, (3) uneven tendon stretching, which results in even more unbalanced finger movement over time, and (4) large unbalanced forces on the object during the grasping process (observed in robotic hands). Also, significant challenges arise in current tendon-transfer surgery due to choosing from a limited set of donor muscles. If the surgeon makes even a 5% error in tensioning the tendons finger movement would be either premature or delayed during the grasping process.
In knee-replacement surgery, the knee joint is replaced with a single-degree of freedom artificial joint that mimics the knee's kinematics. However, knee joint strength typically decreases by 30% following surgery. Such strength loss impedes daily activities such as chair rising and stair climbing.
Accordingly, there is need in the art to develop mechanisms and methods for their use for improving musculoskeletal function. There is a particular need to develop mechanisms and methods that allow for more preferential enhancement, scaling, and/or distribution of a muscle's force and movement across the tendons.